1. Field of the Invention
The present invention relates to a deposition method for producing a thin film of composite material, such as a protective film for a magneto-optical recording medium.
2. Description of the Prior Art
Conventionally, composite material films are produced by sputtering using a single target containing essential components of the film to be formed or by cosputtering using a plurality of targets of respective components of the film to be formed. In the former method, it is necessary to previously prepare a sputtering target of the composite material in a vacuum at a high temperature, and in addition the sputtering target should be produced while considering the inevitably-occurring difference in composition between the deposited film and the sputtering target, the difference varying according to the conditions of sputtering. The cosputtering method is disadvantageous in that the substrate must be located within a relatively narrow and restricted area where necessary sputtering conditions from all of the targets are satisfied, and in that deposition efficiency and filming rate are reduced because of sputtering out of the substrate.
There have been proposed three types of magneto-optical recording materials: (1) rare earth-transition metal amorphous thin films, (2) Heusler alloy film, and (3) oxide magnetic films composed of materials such as garnet and ferrite. They are formed by LPE, CVD or sputtering methods. However, the type (1) material is too readily oxidized and does not have the necessarily adequate magneto-optical effect. Types (2) and (3) materials include those which are hardly oxidized and have large magneto-optical effect, but they are crystalline materials, which are low in film-forming rate, require high heat-treatment temperature, and require an expensive mono-crystal substrate.
When a recording film formed on a plastic substrate such as polycarbonate is made of such a material such as a rare earth-transition metal alloy or a chalcogenide alloy, which is relatively easily oxidized or eroded, it is necessary to cover both surfaces of the recording film with protective films. In other words, recording disks are of a sandwich structure in which the recording medium layer is placed between the protective films. Further, to increase the light absorption efficiency and recording sensitivity of the recording material or to increase the Kerr rotation angle, the protective film on the side exposed to the light beam is made of a material having a high refraction index, and its thickness d is set to satisfy d=.lambda./4n (where .lambda.: wavelength of the light beam, n: refractive index of the protective film material).
These protective films are conventionally made by vacuum-evaporation of ZnS having a refractive index of about 2.3, sputtering of ZnS or an oxide such as SiO.sub.2 in Ar gas atmosphere, sputtering from SiN target in Ar gas atmosphere, or reactive sputtering from Si target or SiN target in N.sub.2 or Ar gas atmosphere. By sputtering of silicon nitride or silicon nitride oxide to a plastic substrate, a dense film is formed which has a small permeability to gas providing a good protective effect, a low oxygen content, and a refractive index of 1.95, but on the other hand which is inferior in cracking-proofness. Hence optical disks provided with such a film are easily damaged by repeated cycles of recording and erasure or by a sudden change in ambient temperature. In other words, they cannot satisfy the required protective effect, recording sensitivity and mechanical strength at the same time.
The method by electron beam vacuum deposition or sputtering of SiO.sub.2 (n=1.5) is disadvantageous in low recording sensitivity and in promoted oxidation of the recording film due to containing of free oxygen at the boundary between the recording film and the protective film during deposition. In particular, the magneto-optical film may undergo changes in composition due to selective oxidation of rare earth element components, accompanied by changes in coercive force as time passes, which causes changes in the shape of recorded bits and/or increases in modulation noise.
The method by sputtering or vacuum deposition of ZnS (n=2.3) is involved in relatively rapid erosion of recording medium due to the permeability of the plastic substrate to water and oxygen. For instance, a TbFeCo film formed between films of ZnS of 90 nm in thickness on a polycarbonate substrate demonstrated that small spots were caused in it at 80.degree. C. and relative humidity of 80% already after 700 hours passed.
As described above, none of the conventional methods of forming protective films satisfies simultaneously all of the following requirements:
(1) To form films which is chemically stable and which have such a low permeability to water and oxygen that sufficient protective effect is provided;
(2) To form films having sufficient mechanical strength against repeated recording and erasure and changes in use conditions;
(3) To form films having higher refractive indexes than 2; and
(4) To be of high deposition rate, good reproducibility of the film properties, and high productivity.